The code parsing register lists involves an inner loop on
each range defined by the lo,hi bounds. The condition on
this loop (for lo<=hi) is fragile, because the bounds
are unsigned 16 bits numbers.

In some corner cases, the calculated upper bound is 2^16-1
leading to an infinite loop.

Parsing operand `[):[o-FP` results in:
- an infinite loop for non ARM architectures
- the generation of almost 2^16 errors for the ARM architecture
  (which are then ignored)

This CL improves the code in 3 ways:
- bail out early when parsing non R prefixed registers
- make sure the register index is never negative
- make sure the number of iterations is limited by the
  maximum size of the range (as a defensive measure).

Fixes #12469

Change-Id: Ib1e7e36fb8ad5a3a52c50fc6219d3cfe2b39cc34
Reviewed-on: https://go-review.googlesource.com/14314Reviewed-by: Rob Pike <r@golang.org>